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Conducted emissions issue with multiple Mean Well AC/DC converters

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prasimix:
I started the EMI testing procedure of the EEZ Bench Box 3 which is carried out in an accredited lab. I was informed that the results of the conducted emissions were negative and that I should try to correct it.
I was in the lab today and got a slightly better picture of the situation, and the situation is complex. Here's what it's about. On mains voltage I have the following Mean Well AC/DC converters: 2 x IRM-10 (for +5 V and +12 V for 10 W each) and up to 3 x LRS-150F (for + 48V / 155 W) but I tested with only one. Mean Wells have some input filters, and the simplified wiring diagram and "conducted emissions" results look like this:



It is important to note that the MCU module is not a simple ohmic load. The picture is similar if the LRS converter is removed from the scheme and only IRMs remain. The first suspect is a 5 V IRM that powers the MCU module. For this reason, I tried to insert a filter in front of the IRMs, temporarily removed the LRS and it gave positive results:



However, the luck was short-lived because as soon as I added the LRS converter even without any load the situation got worse and the test did not pass:



Since the LRS has filter at the AC input, I also tried to connect it directly to 230 Vac and leave the IRMs behind the filter, but that didn’t help. At one point I doubted the CE certifications of the Mean Well module, so to dispel that doubt I connected them directly to 230 Vac, and loaded both IRMs with ohmic loads. The results are then good:



My conclusion is that an IRM loaded with a non-ohmic load combined with an LRS even without a load generates the problem. All the filters I prepared (6 of them) failed to fix this issue. I don’t know if it’s because they all had CM chokes with insufficiently high inductance (below 2.2 mH). I also had a seventh filter that had a CM choke of 2 x 40 mH but unfortunately it turned out that one was not working (one of its lines is broken).

Your input and suggestions are welcome. Many thanks in advance.

RoGeorge:
I'm not very sure this will work, but I'll try to put some resistors at the output of each Meanwell power source (as a constant load).  My assumption is the current either varies too wildly, or it's too small.  Those are switching power supply, they try to keep the required voltage by altering the switching frequency or the on/off ratio, it depends on their type.

If they already have a constant load, the real load won't affect the switching frequency/period so much.  Also, a switching power supply will struggle with no load, sometimes it might not even work without a minimum load.  Either way, an extra resistor or a small light bulb to ensure a minimum current should help.

No idea if this is the main cause, but at least it's easy to test.  Put some load on all the Meanwell sources and see if the noises decrease enough to pass the tests.

jbb:
Like RoGeorfe said, some minimum load might help.

If Mean Well supplies are base on flyback (very traditional for an off line supply, nothing wrong with it) they will be in Discontinuous Conduction Mode (DCM) at low loads. In DCM the switch nodes will likely exhibit some ringing. Try loading things up to 50% load and see what happens...

A possible extra complication; when the assorted supplies are paralleled up, their EMC filter caps are somewhat in parallel, which may shift filter resonances around.

jkostb:
I suspect that the conducted emission issues are caused by common mode currents (L/N -> MCU -> PE). This is confirmed by your experiment where you added a net filter consisting of a common mode choke.

The problem with adding an unloaded ACDC adapter is that the combination of netfilter and unloaded AC DC adapter can cause stability issues. You always need to ensure that the output impedance of the netfilter is lower than the input impedance of the regulator (which in your case is unloaded). When the power supply gets instable you can get conducted emission issues!

Have you tried to add a common mode choke just before the MCU module (and remove net filter) before the meanwell power supplies? I would also add some ceramnic caps over the V+/V- lines to the MCU (100nF) for additional diiferential filtering.

Lastly Meanwell power supplies are very cheap, but you get what you pay for. I know that some Meanwell power supplies have inadequate netfiltering especially for reactive load. If your MCU module is properly filtered you can always try ACDC modules from other manufacturer (TDK-Lambda)

trobbins:
The MCU is shown with an internal PE link to negative supply rail.  Can that link be modified to provide a lossy high-frequency impedance, but still provide an acceptably low galvanic resistance and protective current conduction capability (which I presume is a requirement)?

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